Polyimide exhibits heat resistance, mechanical properties and electrical properties superior to those of other typical resins or engineering plastics, and is thus efficiently utilized in the fabrication of products requiring high heat resistance, including electric and electronic parts. Due to the properties thereof, polyimide resins have been employed in a variety of fields including those of advanced heat-resistant materials, such as automotive materials, aircraft materials, spacecraft materials, etc., and electronic materials such as insulation coating materials, insulating films, semiconductors, electrode protective films for TFT-LCDs, etc. Recently, such a resin is used for display materials such as optical fibers or liquid crystal alignment layers, and is also used for transparent electrode films, either in a manner in which it is contained along with a conductive filler in the films or in a manner in which it is applied on the surface thereof.
Typically, polyimide is synthesized in a manner in which dianhydride and diamine are polymerized in the presence of a solvent to give polyamic acid, which is then heated so as to be dehydrated and cyclized, or in a manner in which a chemical dehydration process is performed using a dehydrating agent so that dehydration and cyclization are carried out.
A polyimide film is manufactured based on the principle of synthesis of polyimide, and is specifically formed through a casting process in which a polyimide precursor, namely a polyamic acid derivative, is applied on a carrier plate and then cured. The casting process includes applying a resin solution on a carrier plate, drying the resin to remove the solvent from the resin, and imidizing the polyimide precursor resin to prepare polyimide.
Meanwhile, polyimide is superior in properties such as heat resistance when manufactured into a film, but is difficult to form into a film due to its rigid rod structure, and is easy to break, making it difficult to perform the manufacturing process. In particular, when a composition comprising p-phenylenediamine and pyromellitic dianhydride is applied on a support and thermally treated, it may foam, may be difficult to form into a film, or may not be separated. When the polyimide film undergoes changes in temperature at high temperatures, it may shrink or expand due to the properties of the film. Here, the extent of shrinkage or expansion of the film is not constant, and thus the use thereof is limited in fields requiring thermal dimensional stability.
Thus, when a polyimide film is used as a substrate for a display device that is recently actively useful, it has to possess thermal stability during high-temperature processing. In the case of a glass substrate for typical use in a display substrate, the coefficient of thermal expansion thereof is approximately 4 ppm/° C., and in order to replace the glass substrate, the polyimide film should satisfy a coefficient of thermal expansion of 10 ppm/° C. or less.
Techniques for the coefficient of thermal expansion or thermal dimensional stability of polyimide include Korean Patent Application Publication No. 10-2012-0073909 (entitled “Polyimide film having excellent high temperature stability and substrate for display device using the same”) and International Patent Publication No. WO2010/113412 (entitled “Low-thermal-expansion block polyimide, precursor thereof, and use thereof”).